The present invention relates to cleaning units and, more particularly, to a cleaning unit provided with a toner drop preventive sheet for receiving residual toner scraped down by a cleaning blade.
In an image forming apparatus recording sheets are fed from a sheet feed cassette or a manual feed tray and then transported to an image forming section by means of transport rollers after having been separated one from another by a separating member. During the process of transporting such a recording sheet to the image forming section, short fibers of cellulose for example intertwining with bleaching agent such as SiO2 (kaolin) and other inclusions are separated from the recording sheet as paper powder on the recording sheet by friction between the recording sheet and the separating member. Such paper powder on the recording sheet is attracted to the surface of a photosensitive drum by an electric field generated in the transfer process and then accumulated on a toner drop preventive sheet included in a cleaning unit.
Since such paper powder has a non-uniform distribution of static build-up, accumulation of paper powder on the toner drop preventive sheet exercises an adverse effect on the removal of residual toner on the photosensitive drum by the cleaning unit, thus causing a cleaning failure and a non-uniform distribution of static charge potential over the photosensitive drum.
When accumulated on the toner drop preventive sheet, the aforementioned short fibers intertwine with each other to form paper powder comprising large particles. Since the toner drop preventive sheet has elasticity, such large-particle paper powder acts to deform the toner drop preventive sheet.
Waste toner collected within the cleaning unit is likely to scatter within the image forming apparatus by vibration caused by a feed screw included in the cleaning unit, wind pressure caused by rotation of the photosensitive drum, and other factor. Such scattering waste toner soils not only the interior of the image forming apparatus but also each recording sheet being transported with a toner image transferred thereto. Thus, the quality of the image formed on each recording sheet becomes degraded.
In an attempt to solve the above-described problem, there has been proposed a technique wherein a cleaning roller having a higher coefficient of dynamic friction than any one of the surfaces of respective of a feed roller and a separating member pressed against the feed roller for rotation is pressed against the outer peripheries of respective of the feed roller and the separating member thereby removing paper powder adhering to these outer peripheries, as disclosed in Japanese Patent Application Laid-open No. H11-106073.
Another known technique is such that a cleaning unit for collecting residual toner remaining on the outer periphery of a photosensitive drum is provided with a capture brush which contacts the outer periphery of the photosensitive drum and is operative to remove paper powder adhering to the outer periphery of the photosensitive drum when applied with voltage, as disclosed in Japanese Patent Application Laid-open No. 2000-081819.
With the technique disclosed in the former publication, however, the cleaning roller cannot effectively remove paper powder in a state not electrostatically charged.
With the technique disclosed in the latter publication, the capture brush contacting the outer periphery of the photosensitive drum is applied with a voltage close to a saturation static charge potential of the photosensitive drum. This causes the photosensitive drum to deteriorate while undesirably allowing a trouble to occur in the subsequent image formation.
A feature of the present invention is to provide a cleaning unit which is capable of collecting paper powder from a photosensitive drum while preventing degradation in image quality due to paper powder collected from the photosensitive drum and accumulated in the cleaning unit.